STRONTIUM ISOTOPE RATIOS AND BORON ISOTOPIC COMPOSITION OF PERMIAN STRATA IN THE U.S. MID-CONTINENT: INTERPRETING A MARINE GEOCHEMICAL SIGNATURE IN NON-MARINE ENVIRONMENTS OF THE BLAINE FORMATION

The Permian seawater strontium curve (⁸⁷Sr/⁸⁶Sr) records one of the most rapid Sr isotope changes in the Phanerozoic, with the declining Cisuralian-Guadalupian ⁸⁷Sr/⁸⁶Sr trend attributed to decreased continental weathering during prolonged aridity. Indeed, widespread indicators of aridity occur in Permian U.S. mid-continent continental strata, seen as red beds and evaporites, arid-type paleosols (gypsisols and vertisols), eolian sandstone and loessite that was cemented by saline groundwater, flooding-evapoconcentration-desiccation cycles in lake deposits, and fluid inclusion analyses indicating acidic lake/groundwater parent brines and extreme hot temperatures. Although few, if any, open marine deposits occur here, prior studies suggest Sr isotopes record marine values.

In this study, we use ⁸⁷Sr/⁸⁶Sr and δ¹¹B composition of the Guadalupian (Roadian?) Blaine Formation in Kansas and Oklahoma to constrain continental water sources. ⁸⁷Sr/⁸⁶Sr of anhydrite, halite, and Microcodium-bearing pedogenic carbonates in core and outcrop have values that overlap with early-mid Permian seawater. In the Rebecca Bounds core, bedded anhydrite in the Blaine has variable ⁸⁷Sr/⁸⁶Sr values (0.70721 - 0.70726) that trend higher up section, opposite the seawater curve, and bedded halite in the middle-upper Blaine has even more radiogenic values (0.70737 - 0.70747). Some basal Blaine outcrop carbonates are also far too radiogenic for Permian seawater (0.70881 - 0.71007). δ¹¹B of the lower Blaine is ~20‰, increasing to ~25‰ in the upper Blaine; this is consistent with marine values and evolution to more concentrated brines. However, brachiopod δ¹¹B indicates Permian seawater is lower than the Carboniferous, with Blaine values similar to Late Carboniferous - earliest Permian.

Seawater-like ⁸⁷Sr/⁸⁶Sr and δ¹¹B values in Permian continental settings is perplexing. Explanations include: 1) brine evolution from early Permian seawater during decreased weathering; 2) sea-spray transport of strontium to continental settings; 3) aquifer eustasy, wherein groundwater flux (and composition?) is related to sea level; and/or 4) lake water geochemical ranges that allow for elemental and isotopic data, such as in Western Australia saline lakes, to coincidentally overlap with non-evaporated and evaporated seawater.